This invention relates to the demetallization of hydrocarbon charge stocks. More particularly, it relates to the removal of metallic contaminants, especially those of nickel and vanadium, from residual oils.
Various petroleum feedstocks, such as crude petroleum oils, heavy vacuum gas oils, shale oils, oils from bituminous sands, topped crudes, and atmospheric or vacuum residual fractions contain varying amounts of non-metallic and metallic impurities. The non-metallic impurities usually include nitrogen, sulfur, and oxygen; the metallic impurities usually include nickel, vanadium, iron, sodium, copper, zinc, and arsenic. In addition, it is known that most the the metallic contaminants are present as inorganic sulfides, oxides, and water soluble constituents, while the remainder is usually in the form of relatively thermally stable organometallic complexes such as metal porphyrins and derivatives thereof. It is generally conceded by the art that for purposes of demetallization the removal of the organometallics is more difficult than that for the inorganics.
The presence of these contaminants in petroleum feedstocks presents assorted difficulties in the refining process. In most petroleum refineries today, however, the problems associated with the inorganic metallics and non-metallic impurities have largely been resolved. Catalytic hydrotreating or hydrofining operations effectively remove sulfur, nitrogen, and oxygen from petroleum stocks by hydrodecomposition. Inorganic metallics, on the other hand, are generally removed at least in part, by filtration, water washing, or electric desalting operations performed prior to hydrofining. But removal of the organometallics, especially those of vanadium and nickel, is especially troublesome. These components are readily adsorbed by conventional hydrofining catalysts and other catalysts used in cracking and reforming operations, resulting in rapid deactivation thereof.
A general method by which these organometallics can be at least partially removed prior to the above catalytic contacting processes is to pass the desalted feedstock (especially those with metals content in excess of 150 ppm) over catalytic adsorptive materials. Many such materials are known in the art. For example, spent cobalt-molybdate hydrotreating catalysts have been used for this purpose, but generally they are too badly coked to provide effective service. Bauxite has also been used for this purpose, as reported in U.S. Pat. No. 2,687,985. And manganese nodules removed from the sea floor have been reported in a series of patents (see U.S. Pat. Nos. 3,772,185, 3,716,479, 3,766,054 and 3,813,331) as being effective catalytic adsorbents for demetallizing petroleum feedstocks.
Although the aforementioned catalytic adsorbents are effective for demetallization purposes, their use usually results in one or more disadvantages. For example, because these catalysts are usually of low capacity, frequent shutdowns for replacement of catalyst are necessitated. Also, to decompose the metal porphyrins into forms adsorbable in the catalyst, high operating temperatures, i.e., above 900.degree. F., are required. This, of course, not only results in high heat energy requirements for the process, but also causes deactivation of the catalysts by coking, a problem which is usually avoided by using hydrogen and elevated pressures. But this in turn simply increases the overall cost of operation, primarily because the amount of hydrogen consumed to prevent coking is substantial. The net effect, therefore, of demetallizing with catalytic adsorbents is that high costs are incurred in a process designed to remove an extremely small proportion of contaminants from a feedstock.
Of course, for feedstocks which are destined to be hydrocracked, demetallizing under severe hydrocracking conditions may prove economical despite the cost of demetallizing with catalytic adsorbents. But for feedstocks which would ordinarily be sent directly to a catalytic cracking unit, or for heavy residua and the like, which would ordinarily be sold as a fuel for power plants and the like, the operating costs of using hydrocracking conditions is uneconomical, primarily because of the costs involved in consuming hydrogen. Thus, for such feedstocks, a process is required for removing nickel and vanadium contaminants that would cause deactivation of cracking catalysts, or would cause corrosion on the external side of power plant boiler tubing, but at the same time not require the undesirable and severe hydrocracking conditions necessary in prior art processes.